Motion tracking or motion capture is a method of tracking a motion of a human body and is technology used for various fields such as film or animation production, sports motion analysis, medical rehabilitation, etc.
As a motion tracking method, there is an optical method and a magnetic field method. In the optical method, a reflection marker is attached to a human body and an infrared ray is irradiated onto the reflection marker by using an infrared camera so as to receive a light reflected from the reflection marker. Also, in the magnetic field method, a magnetic member is attached to the body of a user and when the user moves in a field where a magnetic field is formed, a motion of the magnetic member is determined by a change in the magnetic field. Recently, since a sensor is made small and light due to the development of micro-electromechanical systems (MEMS) technology, a method of using an inertial measurement unit (IMU) has been introduced. The IMU is an apparatus for measuring an inertial force acting on an object moved by an applied acceleration. By measuring translational inertia, rotational inertia, terrestrial magnetism, etc. of an object to be measured, various motion information such as acceleration, speed, direction, distance, etc. of the object may be provided.
FIG. 1 illustrates a state in which a plurality of inertial sensors S are attached to a human body which has joints and body parts. FIG. 2 is a view schematically illustrating a kinematical model of the human body illustrated in FIG. 1. FIG. 3 is a view for explaining coordinate systems used for the kinematical model of FIG. 2. As illustrated in FIG. 2, a human body may be represented as a kinematical model having joints J1, J2, and J3 and body parts 1, 2, 3, and 4 that are rotatable with respect to the joints J1, J2, and J3. The inertial sensors S1, S2, S3, and S4 are not attached to the joints J1, J2, and J3, but to the body parts 1, 2, 3, and 4 such as the trunk, an upper arm, a lower arm, and the pelvis, respectively.
Thus, information regarding a motion of a human body may be obtained by using the inertial sensors S1, S2, S3, and S4 attached to the body parts 1, 2, 3, and 4. A motion of a human body may be tracked by processing the information.
However, when the inertial sensors S1, S2, S3, and S4 are attached to the body parts 1, 2, 3, and 4, as illustrated in FIG. 3, the directions of coordinate systems {A and B} fixed to each of the body parts 1, 2, 3, and 4 need to be accurately matched with the directions of coordinate systems {SA and SB} of each of the inertial sensors S1, S2, S3, and S4. However, since the surface of a human body is uneven, it is practically almost impossible to match the directions of the coordinate systems. Thus, when a motion is to be tracked by using the inertial sensors S, there is a need to match the directions of the coordinate systems.